When it is written well, dialect informs and enhances the reader's sense of place and character. It should never be used for only one reason, for example, to identify a character's race or region. Rather, it should be used when necessary to identify the character's level of intelligence, level of education, background, gender, and current situation. When dialect is written poorly, it will at least interrupt the reading of your work. At worst, it will render your story all but unintelligible.